1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for transmitting a downlink signal in a multiple input multiple output (MIMO) wireless communication system.
2. Discussion of the Related Art
In a mobile communication system, a user equipment can receive information from a base station through a downlink. Also, the user equipment can transmit information to the base station through an uplink. Examples of the information transmitted from and received in the user equipment include data and various kinds of control information. Various physical channels exist depending on types and usage of the information transmitted from and received in the user equipment.
A multiple input multiple output (MIMO) scheme means that a base station and a user equipment simultaneously transmit several data streams spatially by using two or more transmitting/receiving antennas so as to increase system capacity. The MIMO schemes can implement transmit diversity, spatial multiplexing or beamforming by using several transmitting antennas.
The transmit diversity scheme is advantageous in that same data information is transmitted through several transmitting antennas to implement data transmission of high reliability without channel feedback from a receiver. The beamforming scheme is used to increase a received signal to interference plus noise ratio (SINR) of a receiver by multiplying several transmitting antennas by proper weight values. Generally, since uplink/downlink channels are independent in a frequency division duplexing (FDD) system, channel information of high reliability is required to obtain proper beamforming gain. In this case, the channel information is separately fed back from a receiver.
Meanwhile, the spatial multiplexing scheme can be divided into a single user spatial multiplexing scheme and a multi-user spatial multiplexing scheme. The single user spatial multiplexing scheme is referred to as a spatial multiplexing (SM) or single user MIMO (SU-MIMO). In the single user spatial multiplexing scheme, a plurality of antenna resources of a base station are allocated to a single user (user equipment), and capacity of a MIMO channel increases in proportion to the number of antennas. Meanwhile, the multi-user spatial multiplexing scheme is referred to as spatial divisional multiple access (SDMA) or multi-user MIMO (MU-MIMO). In the multi-user spatial multiplexing scheme, a plurality of antenna resources of a base station or radio spatial resources are distributed to a plurality of users (user equipments).
Examples of the MIMO scheme include a Single CodeWord (SCW) scheme and a Multiple CodeWord (MCW) scheme, wherein the SCW scheme is intended to transmit N number of data streams at the same time by using single channel encoding block, and the MCW scheme is intended to transmit N number of data streams by using M (M is always smaller than or equal to N) number of channel encoding blocks. In this case, each channel encoding block generates independent codewords, each of which is designed to enable independent error detection.
Meanwhile, a downlink control channel can include a control signal that defines resource allocation and transport format in respect of a signal transmitted through a downlink data channel. In the 3GPP LTE (long term evolution) standard, the downlink control channel and the downlink data channel are referred to as a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH), respectively, and control information transmitted through the PDCCH is referred to as downlink control information (DCI).
The base station determines a PDCCH format in accordance with DCI intended to be transmitted to a user equipment, and adds cyclic redundancy check (CRC) to control information. The CRC is masked with a unique identifier (referred to as radio network temporary identifier (RNTI) in accordance with an owner or usage of the PDCCH. If the PDCCH is for a specific user equipment, the CRC is masked with a unique identifier of the user equipment, for example, C-RNTI (Cell-RNTI). If C-RNTI is used, the PDCCH carries control information for the corresponding specific user equipment. If another RNTI is used, the PDCCH carries common control information received by all user equipments or a plurality of user equipments within the cell.
After CRC is added to the DCI, channel coding, rate matching and modulation are performed, and the modulated symbols are mapped with physical resource elements and then transmitted to the user equipment. The user equipment monitors a plurality of PDCCHs per subframe. In this case, monitoring means that the user equipment tries to decode each of the PDCCHs in accordance with a DCI format which is monitored. In a control region allocated within a subframe, the base station does not provide the user equipment with information as to where the corresponding PDCCH is. The user equipment discovers its PDCCH by monitoring a collection of PDCCH candidates within the subframe. This will be referred to as blind decoding. For example, if the user equipment performs de-masking for its C-RNTI in the corresponding PDCCH and does not detect any CRC error, it detects a PDCCH having its DCI. The user equipment can be set semi-statically through upper layer signaling to receive PDSCH data signaled through the PDCCH subject to various transmission modes.
In order to receive a downlink signal, the user equipment receives a downlink resource on the PDCCH. If the user equipment successfully detects the PDCCH, it reads DCI on the PDCCH. The user equipment can receive downlink data on the PDSCH by using downlink resource allocation within the DCI.
In downlink signal transmission according to the aforementioned MIMO scheme, the base station can transmit control information as DCI through the PDCCH, wherein the control information is intended to allow the user equipment to normally receive a downlink signal. In case of downlink signal transmission that uses a new MIMO scheme different from the existing MIMO scheme, a problem may occur in that the user equipment fails to normally receive a downlink signal through control information subject to a DCI format defined previously. Accordingly, in a new downlink MIMO transmission scheme, it is required that control information required to normally receive a downlink signal should be provided to the user equipment.